1. Field of the Invention
The invention relates to a method for synthesis and formation of a digital hologram for use in microlithography with a divergent beam in synthesis of the hologram and a convergent beam in image restoration from the hologram.
2. Description of the Related Art
Lithography and, in particular, photolithography is a well-known technique in semiconductor and printed circuit board (PCB) manufacture for creating electrical components and circuits. Photolithography involves placing a mask in front of a substrate, which has been covered by a layer of photoresist, before exposing both a mask and a substrate to light. The areas of photoresist that are exposed to light react and change chemical properties. The photoresist is then developed in order to remove either the exposed portions of photoresist for a positive resist or the unexposed portions for a negative resist. The pattern formed in the photoresist allows further processing of the substrate, such as, but not limited to, etching, deposition, or implantation.
Known in the art is an optical projection microlithography which is the technique that nowadays is most widely used and provide optimal parameters needed for production, e.g., of integrated circuits (IC). An alternative method is a holographic lithography which is based on image formation with the use of a special holographic mask and a laser beam of high coherency. Advantages and disadvantages of both methods are disclosed with more details, e.g., in U.S. patent application Ser. No. 14/142,776 filed on Dec. 28, 2013 by V. Rakhovsky.
U.S. patent application Ser. No. 14/142,776 discloses a method of manufacturing a holographic mask capable of producing an image pattern that contains elements of a subwavelength size along with decreased deviations from the original pattern. The original pattern is converted into a virtual electromagnetic field and is divided into a set of virtual cells with certain amplitudes and phases, which are mathematically processed for obtaining the virtual digital hologram. The calculation of the latter is based on parameters of the restoration wave, which is used to produce the image pattern from the mask, and on computer optimization by variation of amplitudes and phases of the set of virtual cells and/or parameters of the virtual digital hologram for reaching a satisfactory matching between the produced image pattern and the original pattern. The obtained virtual digital hologram provides physical parameters of the actual digital hologram that is to be manufactured.
However, U.S. patent application Ser. No. 14/142,776 does not teach how to synthesize the hologram and restoration of an image from the hologram with reference to the shape of the light beams used in virtual and actual manipulations of the optical beams, especially under conditions when the features of the hologram mask are too small for patterning and therefore are beyond the technological abilities of the conventional methods.